Abstract
Male and female gametes differing in size—anisogamy—emerged independently from isogamous ancestors in various eukaryotic lineages, although genetic bases of this emergence are still unknown. Volvocine green algae are a model lineage for investigating the transition from isogamy to anisogamy. Here we focus on two closely related volvocine genera that bracket this transition—isogamous Yamagishiella and anisogamous Eudorina. We generated de novo nuclear genome assemblies of both sexes of Yamagishiella and Eudorina to identify the dimorphic sex-determining chromosomal region or mating-type locus (MT) from each. In contrast to the large (>1 Mb) and complex MT of oogamous Volvox, Yamagishiella and Eudorina MT are smaller (7–268 kb) and simpler with only two sex-limited genes—the minus/male-limited MID and the plus/female-limited FUS1. No prominently dimorphic gametologs were identified in either species. Thus, the first step to anisogamy in volvocine algae presumably occurred without an increase in MT size and complexity.
Highlights
Unicocca mating-type locus (MT) included two inverted syntenic blocs of 9 and 7 collinear gametologs, and a single gene, CRB1, that was outside these two syntenic blocs (Fig. 2a)
We have illuminated here the initial transition of the sexdetermining chromosomal region or MT during the evolution of anisogamy, and shown that anisogamy can evolve without increased MT complexity
Two sex-limited genes that encode the sex-determining factor MID and the gamete recognition factor FUS1 constitute the core of the MT haplotypes in both isogamous and anisogamous volvocine algae. In this lineage the transition to anisogamy likely involved direct modification of the sex determination pathway controlled by MID rather than by acquisition of new gamete size control genes in MT
Summary
The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. Volvocine green algae are a model lineage for investigating the transition from isogamy to anisogamy. We focus on two closely related volvocine genera that bracket this transition—isogamous Yamagishiella and anisogamous Eudorina. We generated de novo nuclear genome assemblies of both sexes of Yamagishiella and Eudorina to identify the dimorphic sex-determining chromosomal region or mating-type locus (MT) from each.
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